Differential piston for recirculating gas blast interrupters



p 1959 A. s. CASWELL ETAL 2,905,793

DIFFERENTIAL PISTON FOR RECIRCULATING GAS BLAST INTERRUPTERS Filed June 21, 1957 3 Sheets-Sheet 1' m ll lu INVENTORJ u Z m; e 3 m JD M i a WM we 4d Sept. 22, 1959 s, c sw ETAL 2,905,793

DIFFERENTIAL PISTON FOR RECIRCULATING GAS BLAST INTERRUPTERS Filed June 21, 1957 s Sheets- Sheet 2 A. s. CASWELL ETAL 2,905,793

DIFFERENTIAL PISTON FOR RECIRCULATING GAS BLAST INTERRUPTERS Filed June 21, 1957 Sgpt. 22, 1959 3- Sheets-Sheet 3 a Wm M oiw 2 T6 m m1 Mi in Z a M M m United States Patet DIFFERENTIAL PISTON FOR RECIRCULATIN GAS BLAST INTERRUPTERS Arthur S. Caswell, Glenside, and Joseph D. .Wood, Wayne, Pa., assignors to I-T-E Circuit Breaker Company, Philadelphia, Pa.,-a corporation of Pennsylvania Application June 21, 1957, Serial No. 667,204

7 Claims. c1. 200-148) This invention relates to a circuit interrupters of the gas blast type and more particularly to a circuit interrupter having a recirculating gas system wherein the cooperating contacts are operated both into and out of engagement by means of gas pressure without the use of springs for either operation.

When a circuit interrupter protecting a current carrying line is opened, an electric current are is drawn between the parted contacts. At this time rapid interruption of the arc and the prevention of restrike once interruption has taken place are of prime consideration.

A gas blast directed at the arc hastens the deionization thereof by cooling the are and exhausting thehot arcing products from the region between the parted cooperating contacts. All other factors being equal, the larger the gap between the parted contacts the lesser the'likelihood of arc restrike, But a largergap necessarily means a greateroverall size for the interrupter. However, by raising the pressure of the gas in the gap, the separation distance between the parted contacts 'may be reduced without danger'of restrike at a given voltage. That is, the ability of a dielectric gas to resist breakdown at a given voltage gradient increases as the gas pressure increases except at pressures considerably below atmosmove arcing products thereby assuring a continuous supply of gas in'which the dielectriestrength, cooling properties, and interrupting ability is at a'maximum.,

Gas blast interrupters of the priorart usually stored high pressure gas in a tank formingpart of the interrupter structure. 3 At the instant of contact separation, orimmedia'tely prior'thereto, a valve operatively connect-' ed to the contacts was opened to introduce the highpressure gas in the gap between the parted contacts. The introduction of high pressure gas was accompanied by turbulence which decreased the gas flow rate along the arc stream thereby preventing-a rapid arc extinction. In interrupters employing a bridging'member moving transversely with respect to the stationary contacts, the initial gas blast was required to transfer the are from the bridging'contact to a stationary; contact rather than cool the are thereby further delaying arc extinction.

Part of the blast gas was diverted to open an isolating switch when interruption was complete. The contacts were reclosed by springs as the blast pressure dropped.

Closing the isolating switch once again completed the circuit.

In some prior art constructionssuch asthe gas blast Patented Sept. 22, 1959 ice 667,201, filed June 21, 1957, and assigned to the assignee of the instant invention, the interrupter is opened by high pressure gas acting against the force of closing springs which constantly urge the cooperating contacts toward engagement. Means must be provided to latch the cooperating contacts in the disengaged position so that a loss of gas pressure or control circuit power will not be eifective to permit the closing springs to return the cooperating contacts to their engaged position.

The instant invention eliminates the necessity of providing a latch means or other safety device to prevent unwanted operation of the interrupter. This is accomplished by the elimination of the closing springs and the provision of means whereby the cooperating contacts are operated between their engaged and disengaged positions exclusively by gas pressure.

The circuit interrupter of the instant invention includes a separate interrupter unit for each current carrying line of the electrical system being protected. Each interrupter unit comprises a tank having stationary contacts entered through the opposite ends thereof and a movable contact structure positioned between the stationary contacts in axial alignment with the stationary contacts to bridge 'a gap therebetween. The movable contact structure comprises a cylinder having two pistons slidably disposed therein with each carrying a movable contact.

The cylinder includes a large diameter central section and small diameter sections at either end while each piston comprises a first member having a large diameter and a second member having a small diameter. The pistons are positioned in back to back relationship with a chamher being formed at the center of the cylinder between the two large diameter members of the pistons. Intermediate the first and second members is a third member having a lesser diameter that the second member so that a space is formed between the outside surface of the third member and the inside surface of the cylinder. A low pressure reservoir is at all times connected to the space while the interior of the interrupter tank is at all times directly connected to a high pressure reservoir or the output of a compressor, so that the movable and stationary contacts are always surrounded by high pressure gas.

The chamber between the pistons is selectively connectible to the high pressure of the interior of the tank and the low pressure of the space between the third piston member and the cylinder. To open the interrupter, the high pressure gas behind the large piston ends is released to a low pressure reservoir. Since the pressure behind the pistons is now very low, the high pressure gas in the tank acting on the fronts of the pistons moves the pistons along converging paths thereby separating interrupter describedin copending application Serial No;

the movable contacts from the stationary contacts to open the circuit.

The movable contacts have central openings therethrough which, together with openings in the chamber, provide a continuous gas passage between the chamber and tank while the movable contacts are moving away from the stationary contacts. In this manner the are drawn as the cooperating contacts are parted will imme diately be acted upon by a smooth flowing blast of gas 'which rapidly deionizes and cools the arc and exhausts the are products to the low pressure reservoir. When the movable contacts have reached their most fully converged position they will be forced against seats projecting from the chamber walls to thereby seal off the gas passage between the chamber and tank to prevent the pressure in the tank from dropping to too low a value. Since there is high pressure surrounding the parted contacts, the gap length therebetween necessary to prevent restrike will be smaller than if the parted contacts were surrounded by gas at atmospheric pressure.

To then close the interrupter, high pressure gas is introduced to the chamber. The pressure on the front and back of each of the pistons is now equal, but the force-on the back is greater than the force on thefront since the first member is of larger size than the second member. The forces due to thelow pressure gas in the space between the third member and cylinder is of negligible effect. Therefore, the pistons" will be forced to move along diverging paths, thereby forcing the movable contacts into engagement with the stationary contacts to close the circuit. A garter spring or similar resilient means, acting transverse to the piston path, is provided to assure high pressure contact between the cooperating contacts when they are in engagement.

Filters are provided between the low pressure reservoir and the compressor to remove the arc product's and thereby assure a supply of clean gas for subsequent interfuptions.

Since high pressure gas always surrounds the engaged cooperating contacts, a smooth *gas' flow will act on the are from the moment of its inception. Smooth gas flow assures maximum interrupting and dielectric properties since the arcingproducts are rapidly carried away from the arcing gap and a fresh supply of clean gas is being continuously introduced into the gap until interruption is completed.

Accordingly, a primary object of this invention is to provide a circuit interrupter of the gas blast type wherein the cooperating contacts thereof are moved between their engaged and disengaged positions by means of gas pressure and without the assistance of biasing springs.

Another object of this invention is to provide a circuit interrupter of the recirculating gas blast type wherein the cooperating contacts are always immersed in high pressure gas thereby permitting smaller creepa'ge distances, better heat transfer, and faster arc interruption.

Still another object is to provide a gas blast circuit interrupter wherein the are drawn between the cooperating contacts on opening is subjected to a smoothly flowing 'gas blast from the moment of arc inception to achieve rapid arc interruption.

A further object is to provide a gas blast circuit interrupter wherein the contact carrying pistons serve as valve members to turn off .the gas blast when the movable contacts have reached their fully disengaged positions and thereby maintain a high pressure in the gas between the parted contacts.

A still further object is to provide a gas blast circuit interrupter wherein the loss of gas pressure or loss of, control circuit power will not result in the operation of the interrupter.

Yet another object is "to provide a differential piston, slidably disposed in a two width piston, having the movable contacts of the interrupter operatively secured thereto.

These and other objects of this invention shall become more apparent after reading the following description of the accompanying drawings in which:

Figure 1 is a side elevation of a single interrupter unit, partially cut away to show the internal construction thereof, with the circuit closed.

Figure 2 is a fragmentary view of the interrupter unit of Figure 1 with the circuit open.

Figure 3 is a section taken through line 3-3 of Figure 2 looking in the direction of arrows 33.

Figure 4 is a schematic representation of a three conductor system protected by the circuit interrupter of the instant invention.

Figure 5 is a section taken through line 55 of Figure 2 looking in the direction of arrows '5-5.

Referring to the figures, interrupter unit 19 comprises an elongated tank 11 which, together with its internal and external components, is symmetrical about the transverse center line of Figure 1. Only one half of the interrupter unit it? will be described in detail, it being.

- permitting garter spring 111 to affect high pressure 0011-,

4 understood that the other half is merely a mirror image thereof.

Tank 11 comprises two cylindrical shells 12 joined at their first ends and having end plates 13 covering their second ends. Either or both shells 12 and end plates 13 may be constructed of insulating or conducting material as design considerations may dictate. Cover 14 fits over maintenance opening 15 in shell 12. with window 16 in cover 14 permitting a view of the interior of tank 11.

Stationary contact 17 is mounted to the end of stud 18 and is positioned within tank 11. through opening 19 in end plate 13 and is centered therein by insulator 20. Annular member 21 is secured to the end plate 13 outside of tank 11 to serve as a seal between insulator 20 and end plate 13. Corrugated bus-hing 22 lengthens the creepage path between stud 18 and the outside of tank 11.

Movable contact structure 25 is positioned in axial alignment with stationary contact 17 by means of insulator standoffs 26'29 which have their first ends abutting the outside of cylinder 30 and their second ends resting in outer ring 31 which is positioned between the end flanges 32 of shells 12 to provide a seal between the sections of tank 11 when they are joined as by bolts 33.

A passage 34 extends through standoff 26, outer ring 31, and a fitting 35' secured to outer ring 31 for a purpose to be hereinafter fully explained.

Differential piston assembly 36, slidably mounted in cylinder 30 comprises a differential piston 37, a movable contact 38 secured to differentia1 piston 37, and a plurality of'circularly arranged contacts 40 urged into wiping contact with the inside wall 39 of cylinder 30 by" spring means 41. Ring member 43 is force fitted to movable contact 38 to be in good electrical contact therewith while fastening means 58 secures plate ring 44 to ring member 43 to provide a guide for contacts 40'as they are biased radially outward from movable contact 38 by compression springs 41 which abut opposite endsv of adjacent contacts 40. Thus, in the closed position of Figure 1 a complete electrical path is formed between stud 18 on one side of interrupter unit 10 and to stud 18 on the other side'of interrupter unit 10 through stationary contact 17, movable contact 38, contact 40, cylinder 30 and the mirror image of these parts housed in the adjacent shell 12.

Baffies 46, positioned near the transverse center line of cylinder 30, are spaced to form chamber 50, at the center of cylinder 30, which communicates with passage 34 through opening 51 in the wall of cylinder 30. Movable contact 38 is a hollow member having a central passage 54 extending through the complete differential piston assembly 36. Baffle opening 55 provides gas passage.

from contact passage 52 to chamber 50 except when differential piston 37 is driven against seat 56 as in the open position of interrupter unit 10 (Figure 3). Longitudinal slots 53, cut in movable contact 38 at its mating end, provide flexibility for contact deflection thereby tact between cooperating contacts 38, 17.

Cylinder 30 includes a central section 101 having a larger diameter than the end sections 10 2 while differential piston 37 comprises a first member 103, having a diameter substantially equal to that of central section. 101, and a second member 104, having a diameter substantially equal to that of end section 102. Intermediate member 105, having a smaller diameter than that of second member 104, is positioned between members 103,

104 to form a circumferential depression 106 in the outer surface of differential piston 30.

The space formed between the outside of piston 37 and the inside of cylinder 30 by depression 106 is connected to low pressure reservoir 65 through cylinder,

opening 107, gas line 108, passage 109 through insulator 28, and gas line 110. Cylinder opening 107 is so positioned that it communicates with depression 106 for all Stud 18 passes positions of differential piston assembly 36. It is neither practical nor economical to maintain a perfect seal between piston 37 and cylinder 30 so that the gas that does leak past must be bled off to the low pressure tank. If the pressure in 106 were allowed to build up dilferential action of piston 37 would be impossible.

With the interrupter unit in the closed position of Figure 1, high pressure gas is present on both the front 58 and back 59 faces of diiferential piston assembly 36. To open interrupter unit 10 blast valve 60 is opened by applying power to coil 61 which attracts plunger 62 to the left with respect to Figure 4 and thereby moves the pilot valve disc 64 off seat 63, against the force of spring 66. Chamber 50 now communicates with low pressure reservoir 65 through gas line 67, connected between fitting 35 and blast valve 60, and gas line 68, connected between blast valve 60 and low pressure reservoir 65.

Now the pressure on front face 58 is high and the pressure on back face 59 is low so that differential piston assemblies 36 will be driven to a converging position where they will rest on seats 56 (Figure 3). As soon as an arc is drawn between movable contact 38 and stationary contact 17 it will be subjected to a smooth flowing gas blast, flowing from tank 11 to low pressure reservoir 65, which cools and deionizes the are bringing about its rapid extinction. The arcing products are rapidly removed from the arcing gap by the gas blast and carried through the movable contact passage 54, chamber 50, passage 34, and gas lines 67, 68 to low pressure reservoir 65. Equalizing slots 53 permit the gas blast to be initiated at the moment blast valve 60 is opened.

By the time differential piston assemblies 36 rest against seats 56 the arc is extinguished. The gas blast is shut off by seat 56 which seals off chamber 50 from contact passage 54. Piston rings 42 in peripheral grooves 42' serve as a substantially gas tight seal between piston 37 and the inside walls of piston 30. High pressure gas is present in the gap between cooperating contacts 38, 17 since tank 11 is connected through fitting 70 and gas line 69 to high pressure reservoir 71, thereby preventing arc restrike.

When blast valve 60 is closed by deenergizing coil 61, interrupter unit 10 will not close immediately since chamber 50 is not as yet filled with high pressure gas. Closing valve 72 must first be opened by energizing coil 73 which attracts plunger 74 and moves valve disc 75 upward off seat 76 against the force of biasing spring 77. At this time high pressure gas line 69 is connected to gas line 39 which in turn is operatively connected to piston chamber 50 thus bringing both faces 58, 59 of differential piston assembly 36 to the same pressure level. However, the force on back face 59 exceeds that on front face 58 since face 59 has a larger area than face 58. Thus differential piston assemblies 36 will be forced along diverging paths until cooperating contacts 38, 17 are in the engaged position of Figure 1.

Stops 114 are provided at the ends of cylinder 30 to limit the outward travel of differential piston assemblies 36. The net force acting on piston 37 through depression walls 112, 113 is negligible since it is due to low pressure gas. It is essential that depression 106 be maintained at low pressure or else any leakage of high pressure gas from tank 11 passed packing 42 into depression 106 will soon equalize .the forces acting on the differential piston 37 and prevent operation of the interrupter from the open position of Figure 2 to the closed position of Figure 1.

Filter unit 83 is interposed between the input of compressor 84 and low pressure reservoir 65 to remove arcing products from the gas after it has interrupted a first arc and before it will be played on a second arc. The output of compressor 84 is connected to both high pressure storage reservoir 71 and high pressure gas line 69 to assure a continuous supply of high pressure gas within tank 11. Safety valves 82 are strategically placed throughout the system to bleed excess pressures back to low pressure reservoir and prevent damage because of excessive gas pressures.

High pressure gas line 69 also has located therein check valves 85, a shut off valve 89, reducing valves 88, and pressure switch 86. Reducing valve 88 controls the pressure level within tank 11 while pressure switch 86 controls the operation of compressor 84 responsive to the pressure in high pressure reservoir 71. Check valves prevent a loss of pressure in tanks 11 even though the pressure 'in high pressure reservoir 71 or compressor 84 should drop below that of tank 11.

.Control cabinet 90 houses pressure gauge 91 and operation counter 94 which is operated by means of auxiliary switch 93 and air valve 92 connected to gas line 39.

The three interrupter units 10 of Figure 4 may be each connected to a separate current carrying line to provide a double break in each line. However, for severe interrupt-ing conditions two or more interrupter units 10 may be serially connected to produce four, six, eight, etc. breaks in a single power line.

The recirculating gas system also contributes in part to a compact construction by supplying non-contaminated gas to the interrupter. factor used in calculating creepage distances within the tank 11 of the interrupter unit 10, tank 11 may be kept to a minimum since dirt and arcing products are not likely to build up on the insulating surfaces to impair their eifectiveness as insulators.

It should be apparent to those skilled in the art that a suitable construction of differential piston assemblies 36 will eliminate the necessity of forming chamber 50 by means of baffles 46. Instead, the back faces 59 of the newly designed differential pistons will form chamber 50 and in the open position of the circuit interrupter, the newly designed difierential piston assemblies will be positioned back to back to act as a closed valve between passage 34 of insulator 25 and central passage 54 of movable contact 38. Thus, a gas blast circuit interrupter of especially compact and simplified construction has been provided. The construction is such that accidental or unscheduled operation of the interrupter is:

prevented. That is, the cooperating contacts are operated both into and out of engagement by means of gas pressure acting without the aid of springs. High pressure gas must be available for interrupter operation. Furthermore, a loss of control circuit power cannot result in interrupter operation since a positive energization of the gas valves is required to affect movement of the cooperating contacts.

In the foregoing, this invention has been described only in connection with preferred embodiments thereof. Many variations and modifications of the principles of this invention within the scope of the description herein are obvious. not by the specific disclosure herein, but only by the appending claims.

We claim:

1. A gas blast type circuit interrupter comprising a tank, a stationary contact and a movable contact operable into and out of engagement with said stationary contact; said stationary and said movable contact being positioned within said tank; said tank being operatively connected to a high pressure source of dielectric gas; a cylinder fixedly secured within said tank and a piston slidably disposed within said cylinder; said piston including a back member, a front member, and an intermediate member between said back and front members; said back member being of a larger diameter than said front member; said cylinder having a first portion,

ber to thereby form a space between the outer surface Because of this, the safety Accordingly, it is' preferred to be bound of said intermediate member and the insidesurface or said cylinder; said space being operatively connected to a low pressure source of dielectric gas; means-selectively connecting the back of said piston to-sa'id high and said low pressure sources; said movable-contact being operatively connected to the front of said piston; said mov able contact being moved into engagement with said stationary contact by means of gas pressure when the back of said piston is connected to'said high pressure source and being moved out of engagement by means of gas pressure when the back of said piston is connected to said low pressure source.

2. A gas blast type circuit interrupter comprising a tank, a stationary contact and a movable contact operable into and out of engagement with said stationary contact; said stationary and said movable contact being positioned within said tank; said tank being operatively connected to a high pressure source of dielectric gas; a cylinder fixedly secured within said tank and a piston slidably disposed within said cylinder; said piston including a back member, a front member; and an intermediate member between said back and front members; said back member being of a larger diameter than said front member; said intermediate memberhavi-ng a smaller diameter than said front member to thereby form a space between the outer surface of said intermediate member and the inside surface of said cylinder; said space being operatively connected to a low pressure source of dielectric gas; means selectively connecting the back of said piston to said high and said low pressure sources; said movable contact being operatively connected to the front of said piston; said movable contact being moved into engagement with said stationary contact by means of gas pressure when the back of said piston is connected to said high pressure source and being moved out of engagement by means of gas pressure when the back of said piston is connected to said low pressure source; said cylinder including a first section having a diameter substantially equal to the diameter of said first member and a second section having a diameter substantially equal to the diameter of said second member; said firt member always being positioned within said first section and said second member always being positioned within said second section.

3. A gas blast type circuit interrupter comprising a tank, a stationary contact and a movable contact operable into and out of engagement with said stationary contact; said stationary and said movable contact being positioned within said tank; said tank being operatively connected to a high pressure source of dielectric gas; a cylinder fixedly secured within said tank and a piston slidably disposed within said cylinder; said piston including a back member, a front member, and an intermediate member between said back and front members; said back member being of a larger diameter than said front member; said intermediate member having a smaller diameter than said front member to thereby form a space between the outer surface of said intermediate member and the inside surface of said cylinder; an opening in the wall of said cylinder positioned to be in communication with said space for all positions of said piston; said space being operatively connected through the opening in the wall of said cylinder to a low pressure source of dielectric gas; means selectively connecting the back of said piston to said high and said low pressure sources; said movable contact being operatively connected to the front of said piston; said movable contact being moved into engagement with said stationary contact by means of gas pressure when the back of said piston is connected to said high pressure source and being moved out of engagement by means of gas pressure when the back of said piston is connected to said low pressure source; said cylinder'including a first section having a diameter substantially equal to the diameter of said first member and a second section having a diameter substantially equal to the diameter =of;said

second member; said first member being positioned 'within said first section and said second member being positioned within :said second section.

4. A gasblast type interrupter unit comprising :a tank, a first and a second stationary contact, a movable contact structure; said first and second stationary contacts being disposed within said tank and axially aligned in spaced relationship with respect to each other; said movable contact structure being positioned between said first and second stationary contacts in axial alignment therewith; said tank being operatively connected to a high pressure source of dielectric gas; said movable contact structure including a cylinder fixedly mounted within said tank and two differential piston assemblies slidably mounted within :said cylinder therein in back to back relationship with a chamber therebetween; said cylinder having a central section of greater diameter than the diameter of the and sections thereof; each of said difierential piston assemblies including a first member, second member and a third member operatively connected between said first and second members; said first member being of substantially the same diameter as said central section; said second member being of substantially the same diameter as said end sections; said third member being of :lesser diameter than sm'd second member to thereby form a space between the outside surface of said third member and the inside surface of said cylinder; said space being operatively connected to a low pressure source of dielectric gas; said chamber being selectively connectible to said high and said low pressure sources; said differential piston assemblies being moved along diverging paths by means of gas pressure to engage said stationary contacts when said chamber is connected to said high pressure source; said differential piston assemblies being moved along converging paths by means of gas pressure to be spaced from said stationary contacts when said chamber is connected to said low pressure source.

5. A gas blast type interrupter unit comprising a tank, a first and a'second stationary contact, a movable contact structure; said first and second stationary contacts being disposed within said tank and axially aligned in spaced relationship with respect to each other; said movable contact structure being positioned between said first .and second stationary contacts in axial alignment therewith; said tank being operatively connected to a high pressure source of dielectric gas; said movable contact structure including a cylinder fixedly mounted within said tank and two difierential piston assemblies slidably mounted with-in said cylinder in back to back relationship with a chamber therebetween; each of said differential piston assemblies including a circumferential depression in its outer surface at a point intermediate the ends thereof; said depression being operatively connected to a low pressure source of dielectric gas; said chamber being selectively connect'ible to said high and said low pressure sources; said diiferential piston assemblies being moved along diverging paths by means of gas pressure to engage said stationary contacts when said chamber is connected to said high pressure source; said differential piston assemblies being moved along converging paths by means of gas pressure to be spaced from said stationary contacts when said chamber is connected to said low pressure source.

6. A gas blast type interrupter unit comprising a tank, a first and second stationary contact, a movable contact structure; said first and second stationary contacts being disposed within said tank and axially alignedin spaced relationship with respect to each other; said movable contact structure being positioned between said first and second stationary contacts in axial alignment therewith; said tank being operatively connected to a high pressure source of dielectric gas; said movable contact structure including .a cylinder fixedly mounted within said tank andatwo differential piston assemblies slidably mounted within said cylinder in back to back relationship with a chamber therebetween; each of said differential piston assemblies including a circumferential depression in its outer surface at a point intermediate the ends thereof; said depression being operatively connected to a low pressure source of dielectric gas; said chamber being selectively connectible to said high and said low pressure sources; said differential piston assemblies being moved along diverging paths by means of gas pressure to engage said stationary contacts when said chamber is connected to said high pressure source; said differential piston assemblies being moved along converging paths by means of gas pressure to be spaced from said stationary contacts when said chamber is connected to said low pressure source; means acting transverse to the axes of said stationary contact to urge said stationary contacts and said differential piston assemblies into high pressure contact when said stationary contacts and said differential piston assemblies are in engagement.

7. A gas blast type circuit interrupter comprising a tank, a stationary contact and a movable contact operable into and out of engagement with said stationary contact; said stationary and said movable contact being positioned within said tank; said tank being operatively connected to a high pressure source of dielectric gas; a member operatively connected to said movable contact; a front face of said member always being subjected to gas from said high pressure source; a depression in an intermediate portion of said member always operatively connected to a low pressure source of dielectric gas; a back face of said member selectively subjected to gas from said high and said low pressure sources; said back face having a larger area than said front face; said member, when in a first position, being moved to a second position by means of said gas when said member is subjected to gas from said high pressure source; said member, when in said second position, being moved to said first position by means of said gas when said member is subjected to gas from said low pressure source; said movable contact being in engagement with said stationary contact when said member is in said second position; said movable contact being in engagement with said stationary contact when said member is in said first position.

References Cited in the file of this patent UNITED STATES PATENTS 2,125,525 Thommen Aug. 2, 1938 2,153,400 Trencham Apr. 4, 1939 2,574,334 Latour Nov. 6, 1951 2,747,055 Forwald May 22, 1956 FOREIGN PATENTS 388,285 Great Britain Feb. 23, 1933 

